The present invention relates to methods of preparing samples of semiconductor integrated circuits for observation with a transmission electron microscope, and methods of analyzing defects by means of such samples.
The transmission electron microscope (hereinafter, TEM) is often used to observe and analyze the causes of faults that occur in integrated circuits, particular in very-large-scale or ultra-large-scale integrated circuits (VLSI or ULSI circuits). Since a VLSI or ULSI circuit may contain several million transistors and other circuit elements, the first step is to isolate the fault to a particular circuit element and mark the fault site. The integrated circuit is normally too large to be placed in the TEM sample holder, so a sample containing the marked site is taken from the integrated circuit. The sample is further prepared for TEM observation by thinning a small region around the marked site to permit penetration by the electron beam in the TEM. Known sample preparation techniques include optical beam induced current (OBIC) analysis to locate the fault, and lapping, dimpling, ion milling, and focused-ion-beam (FIB) processing to reduce the region around the fault site to a thin slice.
A problem with many conventional sample-preparation methods is that, although the region around the marked site is thinned, most of the rest of the sample is left unthinned. When placed in the TEM, the sample can only be tilted at certain restricted angles without having the remaining thick portions of the sample obstruct the electron beam. This restriction on the tilt angle prevents certain desirable types of observation and analysis from being carried out.
A second problem is that the small thinned section may not provide adequate visualization of the structural defect that caused the fault. This is particularly true in samples prepared for cross-sectional observation, since the thin section often does not reveal the full extent of the defect, and sometimes misses the defect entirely.
With plan-view sample geometries, a third problem arises from the layered structure of the sample. With conventional preparation techniques, it is difficult to determine the individual thicknesses of the layers encountered by the electron beam, hence difficult to perform an accurate analysis of the information provided by the TEM observations. In particular, accurate elemental analysis becomes difficult.